This invention relates generally to rotating shafts and more particularly to methods of controlling the critical speed of a rotating shaft.
In most types of machinery, for example gas turbine engines, it is not allowable to operate a rotating shaft at its critical speed, particularly at its first bending mode, because of the potentially high levels of vibration that may result. This requirement often places substantial constraints and compromises on the machine's design.
Prior art turbofan engines use a shaft diameter that for the shaft's length is sufficient to place the shaft bending critical speed above the shaft's maximum operating speed. Other methods of controlling the shaft's critical speed include using multiple shaft bearings such that the shaft's effective vibration length is reduced and the boundary conditions are sufficiently stiff to also place the critical speed above the operating range. The ability to use these design solutions is compromised when the shaft in question runs concentrically through another shaft, since the inner shaft's length and diameter are constrained by the outer shaft and the surrounding “core engine” structure as, for example, is the case with the low pressure shaft or power shaft of a gas turbine engine.
In some cases, such as turboshaft engines, which often have a limited operating speed range (typically 85% to 100% of maximum RPM), it is acceptable to transiently pass through the critical speed to prevent vibrations from reaching destructive levels. The inner shaft of other devices, such as a turbofan engine, operates over a wide speed range (typically 25% to 100% of maximum RPM), and accordingly there is not a practical “transient” window in which to place the shaft's critical speed, thus requiring the shaft's critical speed to be placed above the maximum operating RPM. Since the “core engine” structure limits the minimum shaft length and maximum shaft diameter, it may not be possible to achieve a feasible turbofan design, or the inner shaft requirements may impose significant operating life and weight penalties on the “core engine”.
It is known to use active bearings to change the end support conditions of a shaft in a gas turbine engine. This changes the critical speed of the shaft and allows the critical speed to be moved away from, or “jumped over” an approaching operating speed. However, prior art active bearing design does not change the effective length of the shaft, which limits the amount of change that may be obtained in the shaft's critical speed.
Accordingly, there is a need for an improved method of changing the critical speed of a shaft.